1. Field of the Invention
This invention relates to Terahertz sources and more specifically to a fiber-laser-based Terahertz source through difference frequency generation (DFG) by nonlinear optical (NLO) crystals.
2. Description of the Related Art
Terahertz (THz) sources are finding widespread applications because of their unique absorption or transparency in different materials. Based on their unique absorption lines in many chemical and biological materials, THz sources are very useful in medical imaging and diagnostics (such as oncology, cosmetics, and dental cares), and pharmaceutical applications (such as drug discovery and formulation, and proteomics). THz sources are used in non-destructive testing and security screening because of the capability of the source to see through various materials such as plastics, cardboard and semiconductors. THz sources are also used in military sensing and imaging.
Several technologies have been used to generate THz sources. P. G. O'Shea et al, Science, (2001) describe a free-electron laser that can produce very high-power THz radiation. R. Kohler et al, Nature, (2002) and M. Rochat et al, Appl. Phys. Lett. (2002) describe a THz source based on cascaded quantum wells. E. R. Brown et al, J. Appl. Phys. (1993) and S. Verghese et al, IEEE Trans. Microwave Theory Tech. (1997) describes a THz source that uses a photomixer. E. R. Mueller et al, Proceedings of the Ninth International Symposium on Space Terahertz Technology, (1998) describes an optically pumped terahertz sources that use long-wavelength IR lasers such as CO2 laser to pump a low-pressure molecular gas such as methanol. L. Ives et al, Vacuum Electronics Conference, 2000 describe a backward wave oscillator THz source. T. W. Crowe et al, IEEE MTT Micro. Symp. Dig., (1999) describe a direct multiplied (DM) THz source. B. B. Hu et al, Opt. Lett., (1995) and D. M. Mittleman et al, IEEE J. Slect. Topics Quantum Electron. (1996) describe THz sources for time-domain spectroscopy that are based on electromagnetic transients generated opto-electronically with the help of femtosecond laser pulses.
Recently another technique based on nonlinear difference-frequency generation (DFG) in nonlinear optical (NLO) crystals has been receiving attention (W. Shi et al, Opt. Lett. (2002); S. Yamamoto et al., U.S. Pat. No. 6,738,397). THz sources generated by DFG are coherent and can be widely tunable. However, the size, weight and integration of the reported free-space implementations are an impediment to commercial success. Each of the approaches mentioned above has one or more of the following disadvantageous features: (1) bulky in size and not portable, (2) requires cryogenic cooling, (3) low output power, difficult to scale to high power, (4) spatial incoherent beam, cannot have diffraction-limited output, and (5) no spectral agility.
To meet the growing demand for THz sources, a new technology or innovative implementation of an existing technology is needed that provides for a compact, lightweight, tunable, high power THz source that doesn't require cryogenic cooling. This source would preferably be capable of generating a diffraction-limited output beam as well.